1. Field of the Invention
The present invention relates to the field of trimming glasses, more particularly ophthalmic glasses.
2. Description of Related Art
An ophthalmic glass results from a series of molding and/or surfacing/buffing operations determining the geometry of both convex and concave optical surfaces of said glass, followed by appropriate surface treatments.
The last finishing step of an ophthalmic glass is a trimming step consisting in machining the glass edge or periphery so as to conform it to the required dimensions for adapting the glass to the glass frame in which it is intended to be accommodated.
Trimming is generally carried out on a grinding machine comprising diamond abrasive wheels that perform the machining step as defined hereabove.
The glass is held during such a step by axially-acting clamping elements.
The relative movement of the glass relative to the abrasive wheel is controlled, generally digitally, so as to get the required shape.
As it is obvious, it is absolutely imperative that the glass be firmly held during such a movement.
Therefore, before any trimming step, a glass-holding step is performed, i.e. a holding means or acorn element is positioned onto the convex surface of the glass.
A holding pad, such as a self-adhesive sticker, for example a two-sided adhesive, is arranged between the acorn element and the glass convex surface.
The so-equipped glass is positioned onto one of the above-mentioned axial clamping members, the second axial clamping element thus clamping the glass onto its concave surface by means of an abutment, generally made in an elastomer.
During the machining step, a tangential torque stress is generated on the glass, which can result in the glass rotating relative to the acorn element if the glass-holding system is not efficient enough.
The efficient glass-holding mainly depends on the good adhesion at the interface between the holding pad and the glass convex surface.
The latest generation ophthalmic glasses most often comprise hydrophobic and/or oil-repellent anti-stain surface coatings associated with anti-reflection coatings.
These are most often fluorosilane-type materials that reduce the surface energy so as to prevent adhesive greasy stains which are thereby easier to remove.
One of the problems generated by this type of surface coating is that they achieve such an efficiency that the adhesion at the interface pad/convex surface is thereby altered, even compromised for the most efficient hydrophobic and/or oil-repellent coatings.
It becomes therefore more and more difficult to perform satisfactory trimming steps, more particularly for polycarbonate glasses the trimming of which results in much more important stresses than for other materials.
As a result of an inadequately performed trimming step, the glass is purely and simply ruined.